An efficient means to heat water is to flow hot gas from a burner through a heat exchanger of a hot water heater. The heat energy of the hot gas is transferred to the water, typically by passing either of the water or the heated gas (e.g. flue gas) through a plurality of tubes, and the other of the water or heated gas on the outside of the tubes. One common type of cylindrical heat exchanger has two or more tube sheets to support a plurality of tubes within the cylinder. The tubes, which run lengthwise through cylinder are distributed along the tube sheets.